Method of making radar domes



Oct. 14, 1952 s. COOPER METHOD OF MAKING RADAR DOMES Original Filed May6. 1949 m w W T T A Patented Oct. 14, 1952 METHOD OF MAKING RADAR-DOMESLester S. Cooper, Columbia, Conn, assignor to Rubatex Products, Inc.,New York, N. Y., a corporation of Delaware Original application May 6,1949, Serial No. 91,676. Divided and this application January 3, 1951,Serial No. 204,211

1 Claim.

My present invention is a division of my application Serial No. 91,676filed May 6, 1949 and relates to protective devices, housings, orshields of high permeability to radio frequencies particularly in theultra-high frequency range and more particularly to a protective unitadapted to enclose and streamline a radar antenna mounted in a movingvehicle such as aircraft.

expanded closed cell cellular rubber commonly known as Rubatex has adielectric factor closely approximating that of air and is thus highlypermeable to radio frequencies and, therefore, will not interfere withthe transmission or reception of radar signals.

Heretofore in the manufacture of radar domes which were usuallyspheroidal in shape (being either semi-spherical or projectile shapeddepending on the location in the aircraft) attempts were made to securean eflicient protective dome structure by making the dome of a pluralityof gores cemented to ether.

It was found, however, that the cement lines interfered with thetransmission and reception of the radar signals.

A radar dome is particularly necessary on aircraft as a protectionagainst the vibration and mechanical stress on delicate antennae owingto aerodynamic forces, as well as owing to airborne objects of variouskinds found in the atmosphere.

In addition, where the radar antenna is mounted on the underside of thefuselage, the radar dome is necessary to protect the antenna againststones and dirt which may be thrown up by the airplane wheels whiletaxiing, landing, or taking off.

Accordingly, the radar dome must be rigid and stron while at the sametime it must be substantially transparent to radio frequencies.

The rigidity and strength is obtained by the utilization of hardexpanded closed cell rubber formed in accordance with the methoddisclosed in Patents Nos. Re. 21,245, 2,299,593, and 2,268,621 andformed with sufficient sulphur so that when vulcanized a hard board willresult.

The dome is sandwiched between fibre glass sheets impregnated with analkyd resin to form a rigid structure having the high permeability abovenoted.

When, however, the board was formed from gores cemented together, thehigh permeability of the structure as a whole was nullified when anyparticular radar signal was required to pass through one of the gluelines. False reflections andsignals occurred and at times the signalwasabsorbed.

In addition,.whlle it was desirable that the signal be generated from apoint at the center of the dome, the various shapes required of the domemade it necessary for the signal to traverse the material of the dome atdifferent angles for difierent positions of the antenna varying thethickness of the material traversed.

The object of my invention is the formation of a unitary dome structureof hard closed cell expanded cellular material wherein the material ishomogeneous and continuous throughout without any glue lines.

An additional object of my invention is the formation of the dome asabove set forth in a sandwich construction between fibre glass sheetswherein the single homogeneous dome is securely integrated with thefibre glass sheets and wherein the entire structure is formed and shapedand brought to final completion in a mold.

The molding of the entire dome makes is possible to vary the thicknessesat different areas of the dome so that where the dome is used'as a radardome the variations in thickness of the dome will ensure that for everyangular position of the radar antenna the signal will be required totraverse an equal thickness of the dome.

The foregoing and many other objects of'my invention will becomeapparent in the following description and drawings in which:

Figure l is a schematic view in cross-section showing a piece of solidrubber intended to form the core of my novel dome prepared for moldingto the preliminary shape.

Figure 2 is a schematic view in cross-section showing the solid rubbermolded to the preliminary shape;

Figure 3 is a schematic. view in cross-section showing the pre-moldedsolid rubber placed in a second moldand blown to the precured stagedescribed in the above-mentioned patents.

Figure 4 is a cross-sectional view through the precured stage of thedome as it is removedfrom the mold of Figure 3 in frozen condition,

Figure 5 is a schematic view in cross-section showing the precured domeof Figure 4 placed in the final mold between sheets of fibre glass to beblown to the final shape. I 4

Figure 6 is a cross-sectional view completed dome. 7

Referring now to the figures, the dome I0 is composed of an inner skin Hand an.- outer skin 52 of a combination of glass fabric thermoplast anda core I3 of hard closed cell cellular rubber. The rubber compound mustnot contain anything deleterious suchas metal contamination, car-. bon,etc.

through the Generally, the distance of separation of inner and outerskin is approximately one half inch more or less but may vary in thesame dome at different portions of the dome curve due to complicationsof tangential exit and entry of the electromagnetic radar impulses.

In order to satisfy the dielectric uniformity of the cellular rubbercomponent of the sandwich construction, it is important that all rubbergas cells be of the same size throughout, that there be no voids orbreaks or glue joints and that the rubber core have a uniform densitythroughout. Only then can the strict dielectric specifications berealized.

Heretofore the cellular rubber sheet was cut into gores which wereassembled to the required shape by gluing and then the whole was gluedinto two glass plastic skins. The rubber sheet itself had a tough skinon both surfaces.

The difficulty was that the radar signal suftiered distortion by, theglue joints which acted opaque and the .more so as the signal had topass through complicated patterns tangentially. The rubber skin in ageneral manner reduced effi- .ciency. The gluing had to be accepted forwant of something better.

On the whole, a good balance can be found for Physical strength anddielectric effect with a glasspplastic rubber sandwich because thecellular rubber mentioned has:

'(a) A low dielectric factor,

(1)) Low loss tangent,

to) Is of low density, 8 pound per cubic foot being used,

:(d) Isidealfor sandwiching,

fie) Low water absorption which is of value if outer glass skinisdamaged,

If) Goodprocessa'bility.

The deleterious effects of gore glue joints and skiing to the glassskins as well as the rubber :skins :are done away with in my novel dome.

The process consists of the following steps:

31. Pro-shaping the rubber dough into a domesha-ped solid rubber core ofone-half linear dimensions (Figures 1 and 2) 2. Precuring thedome-shaped solid rubber core 'in a miniature mold of essentially thedimensions and shape 'of'the rubber core of step 1 (Figure 3),

8. Expanding the precured miniature rubber core to slightly .less thanthe glass dome size (Figure 4) i. Freezing the expanded rubber dome, ifnecessary, to theipoint of crystallization.

5. Skinning the inner and outer surfaces of the hardened rubber dome (bychilling) with a suit able knife or .scrapcr to provide an open cellsurface suitable to adjoin the glass skins.

6. Installing this preparation within the two glass skins. The precuredand skinned rubber .core at this stage of the process does notcompletely'fill the inner space of the glass skins but isal'lowedfurther expansion.

"7. Installing this unit within a mold to be steam heated, subsequently,for complete vulcanization of the rubber core. This mold closely fitsthe dome-shaped shield. Mounting this filled mold upon a base plate(Figure 5) :8. Evacuating the remaining space (to be fully filled by therubber core) through the base plate 'KFigure 5) 9. Heating the rubbercore through the glass skins to a final hard rubber vulcanizate.Additional evacuation is employed to remove H28 as forming and to removeany air bells from within the sandwich and, furthermore, to force therubber against the glass skins by virtue of increased internal rubbercell pressure.

In step 1 the molded solid rubber core is, after cooling, diminished atthe :edge approximately five per cent of its weight in order to providespace for thermal expansion of the rubber solid during the precuringperiod.

In step 2 the confined blowing agent gradually reaches a gas pressure of1,500 pounds p. s. i. in consequence of which the gas causes an internalgassing of the rubber. The final gas cells are actually much smallerthan the particle size of the chemical blowing agent admixed to therubber.

A .refinem'ent consists in cooling down the hot procure Within theblowing mold before expansion in order to avoid difiusion of the hot gasfrom the outer layers of the cell rubber and thereby building up aheavy, tough rubber skin. llhis gives a product of less density than canbe had Without cooling.

After the cell rubber has regained its physical strength, it iscarefully expanded by heat to a predetermined size and refrigerated aspointed out.

In step 5 care must be taken to avoid any damage to the open cellsurface by a dull knife and laying down the cell Walls. By moving theknife obliquely or by reciprocating motion and using water freely aslubricant, a satisfactory skinning operation is possible. The knife foroutside use is concave, while the knife for inner use has a convex bend.

The freezing of step 4 not only controlled-expansion but also providedthe necessary consistency .of the cell rubber to be successfullyskinned.

In step -8 the evacuation .is performed before steam is admitted to thejackets of the mold and while the rubber core is still cold andunyielding allowing a complete evacuation of the remaining air space andcreating vacuum cups out of the i open surface cells.

surfaces by virtue of the vacuum.

Referring again :to :the drawings, a solid rubber sheet l-3a which hasbeen mixed with the appropriate amount of sulphur for securing a hardvulcanization and the appropriate amount of chemical material forecuring an appropriate blow or expansion in accordance with theexpansion of the showing of the above patents is placed over the moldcore I5 (Figure 1) and the mold shell ['6 is pressed down thereon asshown in Figure 2.

If necessary, suflicient heat is applied to plasticize the rubber enoughto permit the formation of Figure '2 to be obtained and retained by therubber without causing a blow to occur. Since the rubber [3a is inunvulcanized condition, the plasticizing heat will not ordinarily benecessary.

The pre-formed rubber sheet l3a of Figure 2 is then removed from themold of Figure 2 and placed over the steam heated core I8 as shown inFigure '3.

A steam heated shell I9 is placed over the preformed unvulcanized rubbersheet. The spacing between core I8 and shell I9 is somewhat greater thanthe spacing between core l5 and shell I6 to permit some expansion of therubber.

The mold |8-| 9 is now heated to a degree sufficient to cause partialvulcanization of the rub-' her and a partial decomposition of the crudechemical material to produce a partial blow so that the rubber material|3b in the mold |8|9 is expanded to the precure stage.

If it were removed from the mold |8-| 9 at this time without anypreliminary Operation, it would then expand further diminishing orprecluding any possibility of further working thereof.

However, mold |8-|9 is now chilled to a point where the rubber is almostcrystalline in form and thus cannot be expanded. The chilled precuredcore 13b is now removed from the mold |3-|9.

The precured core |3b of Figure 4 is then subjected to a cuttingoperation with a sharp blade which will remove the outer skin formed bythe collapse of the cells adjacent the mold walls so that a plurality ofminute cups are formed in the inner and outer surface of the core |3b.

A sheet of fibre glass H impregnated with an alkyd resin is then placedover core 2|) of the mold of Figure 5. The refrigerated core 131) isplaced thereover. A second sheet of fibre glass ill impregnated with analkyd resin is placed over the core I31) and the shell 2| of the mold 29is placed over the fibre glass sheet ID.

The spacing between core 2|] and shell 2| is slightly greater than theoriginal thickness of the chilled core |3b and the sheets I0 and IAppropriate tubes, openings, or recesses 25 are provided in core andshell 2| for the admission of steam and appropriate conduits 21, 28 areprovided for exhaust of condensed moisture or of steam.

The shell 2| is secured by means of bolts 30 through lugs 3| to baseplate 32 which is an annular base plate carrying the core 23. One ormore openings 33 are provided in base plate 32 communicating with thespace between shell 2| and core 26, the said openings 33 being connectedto a Vacuum pump.

Air is first exhausted from the space between shell 2| and core 20 sothat the open cells at the surfaces of the rubber core |3b willconstitute vacuum cups. After the air is exhausted, heat is applied bymeans of steam through passages in mold 202| to fully Vulcanize therubber core -|3 and expand the same into close and intimate contact withthe impregnated fibre glass sheets H1 and I.

The internal pressure forces the multiplicity of open minute cells inthe skin of the core into vacuum cup type engagement with the glasssheets, while the alkyd resins liberated by the heat serve to integratethe rubber and glass fibre sheets at their meeting surfaces.

After expansion and final vulcanization, the shell 2| may be removed andthe completed dome is then available for operation.

In the foregoing I have described my invention solely in connection withspecific illustrative embodiments thereof. Since many variations andmodifications of my invention will now be obvious to those skilled inthe art, I prefer to be bound not by the specific disclosures hereincontained but only by the appended claim.

I claim:

The method of forming a compound curved radar shield having a core ofcontinuous homogeneous closed cell expanded sheet material confinedbetween an outer layer and an inner layer of fibre glass sheet materialimpregnated with an alkyd resin comprising the steps of forming anunvulcanized rubber mix into the compound curve; partially expanding andpartially vulcanizing the rubber mix; chilling the partially expandedand vulcanized mix; cutting the surface skin from the partially curedrubber; placing the partially cured rubber between fibre glass sheets,formed into said compound curve evacuating air from between the rubberand the fibre glass sheets; heating the compound unit and fullyexpanding the rubber into intimate contact with the fibre glass sheets,and softening the alkyd resin and integrating the rubber and fibre glassat their meeting surface.

LESTER S. COOPER,

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,247,337 Raflovich June 24, 19412,414,125 h n ank .--=-.--.-e an, 1.

